What is FOUND.000 folder? What are .CHK files stored in it?

Whenever a disk scan and repair tool run, such as ScanDisk or ChkDsk (depending on your operating system), the default action when finding errors on the hard disk (such as invalid clusters) is to save the affected files as a .CHK file, thus "repairing" the error.

If you were lost something important, you might want to try to recover any data by this software from any CHK files that exist on your computer (usually stored within : C:\FOUND.000, FOUND.001... hidden folders).